Industrial process control systems are used to monitor and control industrial processes which produce or transfer materials, such as process fluids. In such systems, it is typically important to measure “process variables” such as temperature, pressure, flow rate, and others. Process control transmitters are used to measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room.
One type of process variable transmitter is a pressure transmitter that measures one or more pressures (such as gauge or absolute pressure) of a process fluid and provides an output related to the measured pressure. The pressure transmitter is configured to transmit the pressure information back to the control room or other suitable device, typically via a wired process communication loop. However, other techniques, such as wireless communication techniques may be used instead or as well.
Process pressure transmitters generally sense pressure using a pressure sensor that is fluidically coupled to an isolation diaphragm. The isolation diaphragm isolates the pressure sensor from process fluids that are being sensed. Process fluids, which can be highly corrosive and/or at high temperatures, are thus kept isolated from the pressure sensor in order to avoid corrosion or damage to the pressure sensor. Pressure is transferred from the isolation diaphragm to the pressure sensor using a substantially incompressible, inert fill fluid, such as silicone oil. The pressure sensor itself has a physical structure such as a sensing diaphragm that reacts to the pressure, such as by deforming. The pressure sensor also includes an electrical structure, such as a strain gage or capacitive plate or electrode that reacts to the physical deformation. For example, some known pressure sensors have a deflectable diaphragm that bears a capacitive plate or electrode such that deflection of the diaphragm produces a change in the sensor's capacitance. Still other pressure sensors employ a resistive strain gage structure that is disposed on a brittle deformable silicon substrate. As the silicon substrate deforms in response to the pressure, the resistance of the strain gage changes.
One of the limitations of existing pressure transmitters is their potential susceptibility to high over pressures. Since, isolator-based pressure transmitters use a substantially incompressible fluid to couple the pressure sensing element to the process fluid or gas, the fill fluid transmits high pressures to the sensing element during an overpressure event. This can result in sensor failure. Moreover, even in situations where the overpressure does not immediately cause sensor failure, repeated excursion to relatively high overpressures can degrade the pressure sensor over time and affect its performance.